The present invention relates to optical imaging devices. The invention can be used in connection with microlithography, e.g. as employed for the production of microelectronic circuits. It therefore furthermore relates to an imaging method.
Particularly in the field of microlithography, besides using components made with the greatest possible precision, it is necessary inter alia to position the components of the imaging device, for example the optical elements such as lenses or mirrors, as exactly as possible in order to achieve a correspondingly high image quality. The high accuracy requirements, which are in the microscopic range of the order of a few nanometers or less, are not least a consequence of the constant need to increase the resolution of the optical systems used for the production of microelectronic circuits, in order to push forward miniaturisation of the microelectronic circuits to be produced.
With increased resolution, and the generally concomitant reduction of the wavelength of the light being used, not only do the requirements for the positioning accuracy of the optical elements used increase. Also, the requirements with respect to minimising the imaging errors of the overall optical arrangement increase.
In order to comply with the stringent requirements for positioning the components involved with the short wavelengths in the ultraviolet (UV) range used in microlithography, for example with wavelengths in the region of 193 nm, but also particularly in the so-called extreme UV range (EUV) with working wavelengths in the range of from 5 nm to 20 nm (usually in the region of 13 nm), it is often proposed to capture the positions of the individual components such as the mask stage, the optical elements and the substrate stage (for example a wafer stage), respectively and individually with respect to a reference (for example a reference structure, which is often formed by a so-called metrology frame) and then to position these components actively with respect to one another. Such a procedure is known for example from U.S. Pat. No. 7,221,460 B2 (Ohtsuka), the entire disclosure of which is incorporated herein by reference.
This solution has on the one hand the disadvantage that, generally, no real time measurement of the position of the image of the projection pattern of the mask on the substrate (usually a wafer) is carried out, but instead the relative position of the components and the position of the image are merely deduced indirectly from the individual position data of the components. In this case, the respective measurement errors add up, so that a comparatively high overall measurement error can sometimes occur. Furthermore, this entails a large number of elements to be positioned accurately, all of which have to be positioned and measured with respect to their position with the corresponding angular accuracy in the nanorad range (nrad) or less and a translation accuracy in the picometer range (pm). This also leads to particularly stringent requirements for the thermal stability of the reference and the support structure for the optical elements. Here, only a few dozens of nanometers per kelvin (nm/K) are generally permissible in respect of the thermal expansion.
On the other hand, a range of solutions is also known in which the quality of the imaging of the object points of an object plane (projection pattern of the mask) onto the image points in an image plane (in which the substrate is to be arranged), particularly the position of the image of the projection pattern in the image plane, is determined in real time. In this case, the imaging quality, particularly the position of the image of the projection pattern on the image plane, can in principle be corrected with far fewer active elements, and sometimes even with just one active element. Not only does this simplify the dynamic driving of the other components, but also much less stringent requirements need to be placed on the thermal stability of the reference and the support structure for the optical elements.
For example, it is known to determine the aberrations of the projection beam path directly, i.e. with wavefront sensors arranged in the image plane at the position of the substrate. To this end, however, the exposure operation of the substrate needs to be interrupted. This is a practicable solution for projection devices which are sufficiently stable as a function of time, that is to say the monitoring and correction of imaging errors can be carried out at sufficiently large time intervals (from a few hours to days or even weeks). Measurement and correction at shorter time intervals, as would be necessary for perturbations acting within a short time (for example thermal perturbations), is either not possible since the measurement would in principle take too long, or is undesirable since the exposure process ought not to be interrupted or the effects on the productivity of the imaging device are unacceptable.
Real time determination of the position of the image of the projection pattern of the mask on the substrate is often carried out according to the so-called laser pointing principle. In this case, a measurement light bundle in the form of a collimated light beam is guided from a light source arranged in the region of the mask in the vicinity of the path of the useful light (i.e. at the image field edge) via the optical elements involved in the imaging as far as into the region of the substrate, and captured there by a detector. Even very small deviations of the optical elements from their setpoint positions then generate a deviation of the light beam from its setpoint position, which is captured by the detector and used for correction. Such a method is known for example from US 2003/0234993 A1 (Hazelton et al.), the entire disclosure of which is also incorporated herein by reference.
Here, owing to the guiding of the laser beam via the optical elements involved in the imaging, it is sometimes not only possible to determine deviations with respect to the correct position of the image of the projection pattern of the mask on the substrate, but moreover other errors (for example distortions etc.) in the imaging can be captured. All these position errors and other errors are combined under the term imaging error in the present description.
These variants for determination of the imaging error, however, often require intervention in the projection beam path (for example by introducing beam splitters) and sometimes entail an undesired loss of radiation power etc.
As already mentioned, the problems explained above in relation to the angular accuracy and the translation accuracy for the positioning and orientation of the components involved in the imaging have particularly great repercussions in imaging devices which operate in the EUV range (for example in the 13.5 nm wavelength range), as is known for example from U.S. Pat. No. 7,226,177 B2 (Sasaki et al.), the entire disclosure of which is also incorporated herein by reference.
These objectives currently operate exclusively with reflective optical elements (that is to say mirrors or the like), which guide the EUV light by reflection, with, mostly, four, six or eight mirrors mostly being used. During operation of the imaging device, these mirrors generally change their position and/or orientation (owing to mechanical and/or thermal perturbations), since the support structure cannot be absolutely stable (mechanically and/or thermally).
The beam path in such an EUV objective may be a plurality of meters long. In the systems known from U.S. Pat. No. 7,226,177 B2 (Sasaki et al.), for instance, the individual spacings of the optical surfaces lead to an overall beam path length of about 2.5 m from the first mirror (M1) to the plane of the substrate to be exposed. In relation to this, a permissible image displacement with structures to be exposed in the region of about 25 nm must also lie only in the nm range. An admissible image displacement of 1 nm in this case entails a maximum permissible mirror tilt of only 0.2 nrad for the first mirror M1. For the subsequent mirrors (M2 to M6) in the beam path, these angular tolerances increase stepwise since their distances from the plane of the substrate become smaller and smaller.
A correspondingly precise measurement (and subsequent correction) of the alignment of such mirrors, however, is not possible with the previously known devices. There are furthermore combinations of mirror tilts whose effects in relation to the overall image displacement cancel out, and therefore do not cause any image displacement. Such mirror tilt combinations nevertheless lead to deviations of the wavefront, that is to say aberrations, which cannot, however, be identified with the previous methods.